The present invention relates to a digital filter which is commonly referred to as a FIR (Finite Impulse Response) filter and is used, for example, to restrict a digital data signal within a desired frequency band in the case of transmitting the signal.
With the use of a method which transmits a speech or image signal in digital form and decodes it to reproduce a speech or image, it is possible to reproduce a speech or image signal of good quality. Hence, attempts are now being made for digitization in the fields of, for example, radiotelephony, portable television cameras, and so forth.
According to a conventional digitization method, an analog signal from a signal source is converted by an A/D converter to a digital signal, the thus A/D converted output is further converted by a parallel-serial converter to a serial pulse train signal, which is provided to a modulator, wherein a carrier signal is subjected to an amplitude, frequency or phase modulation, and the modulated signal is transmitted. At the receiving side the transmitted signal is demodulated to obtain the pulse train signal. The pulse train signal is converted to parallel form to obtain parallel digital data, which is further converted by a D/A converter to digital form, thus reproducing an analog speech or image signal.
If the carrier is modulated with the pulse train signal held intact at the transmitting side, side bands centering about the carrier frequency will develop over a wide frequency range, because the pulse signal contains many higher harmonics. Hence the transmission occupies a wide frequency band. This defect could be overcome by restricting the frequency bandwidth of the modulation signal through use of an analog filter. With the analog filter, however, it is difficult to obtain a sharp attenuation characteristic. Moreover, even if a filter of a sharp attenuation characteristic is formed by, for example, a crystal resonator, the phase rotation will become more and more prominent as the attenuation characteristic becomes steeper.
When the pulse train signal is applied to a circuit of excessive phase rotation, jitters markedly develop owing to the phase rotation, resulting in the deterioration of transmission quality. In view of this, a digital filter has heretofore been employed to restrict the bandwidth of the digital signal. The digital filter is suitable for use as a transmission line for the digital signal in that a steep attenuation characteristic can be. Even if the attenuation characteristic is steep, no phase rotation will occur. Hence, it is customary in the prior art to utilize, at the transmitting side, an arrangement in which the pulse train signal to be input as a modulation signal into the modulator is applied to a digital filter to eliminate higher harmonics contained in the pulses.
In FIG. 1 reference numeral 10 denotes a conventional digital filter, which is made up of an N-stage shift register 11, multipliers 6.sub.0 to 6.sub.N-1 for multiplying the outputs of respective shift stages S.sub.0 to S.sub.N-1 of the shift register 11 by coefficients h(i-n), where i=0, 1, . . . , 2n, and adders 7.sub.1 to 7.sub.N-1 for sequentially adding the multiplied outputs to obtain a total sum of them. An analog signal from a signal source 13 such as a microphone is converted by an A/D converter 14 to a digital signal, which is converted by a parallel-serial converter 15 to a serial data signal S(i), which is then provided to an input terminal 12. The data signal S(i) is input into the shift register 11 bit by bit upon each occurrence of a clock CK and is shifted through the shift stages S.sub.0 to S.sub.N-1.
Pieces of bit data S(O) to S(N-1) in the respective stages S.sub.0 to S.sub.N-1 of the shift register 11 are extracted therefrom and provided to the multipliers 6.sub.0 to 6.sub.N-1 at one terminal thereof. The multipliers 6.sub.0 to 6.sub.N-1 are each supplied at the other input terminal with the coefficient h(i-n) defining an impulse response characteristic shown in FIG. 2. The provision of the coefficient h(i-n) defines, for example, the upper limit frequency of the lowpass band of the digital filter 10 and its cutoff characteristic. The multiplied outputs of the multipliers 6.sub.0 to 6.sub.N-1 are added by the analog adders 7.sub.1 to 7.sub.N-1 to obtain a total sum, i.e. ##EQU1## and an analog signal of an amplitude equal to the value of the total sum is provided at an output terminal 18.
For instance, in the case of forming, as the digital filter 10, a 20-order FIR filter by hardware, one 20-bit shift register 11, 20 multipliers 6.sub.0 to 6.sub.N-1 and 19 adders 7.sub.1 to 7.sub.N-1 are needed and the circuit scale is large. This hardware structure is not particularly suitable for portable communication equipment. On this account, it is a general practice in the prior art to implement the FIR filter by forming the shift register 11, the multipliers 6.sub.0 to 6.sub.N-1 and the analog adders 7.sub.1 to 7.sub.N-1 by a microcomputer.
In the case of the digital filter 10 being formed by a microcomputer, it is hard to speed up its operation, because the microcomputer needs to repeatedly execute as many as 20 steps of multiplications and analog additions each time the data of the shift register 11 is shifted one bit position. Consequently, in the case of forming a lowpass filter, its treble cutoff frequency cannot be set high, that is, a relatively high-speed digital signal cannot be transmitted. Moreover, since the throughput of the microcomputer is consumed in association with the digital filter alone, the time for executing other control programs is reduced, and hence the control function cannot be improved.